1. Field of the Invention
The present invention relates to an image forming method, and more particularly to an image forming method using a contact charging method wherein a charger charges a photoreceptor while contacting thereto.
2. Discussion of the Background
Conventionally, a corona charging method has been used for charging a photoreceptor so as to have a predetermined potential. The corona charging method includes a discharge electrode such as a wire electrode and a shield electrode surrounding the discharge electrode. A high voltage is applied to the discharge electrode and shield electrode to cause a corona shower charging the surface of the photoreceptor.
Recently, a contact charging method which is more environmental resistant and power saving than the corona charging method is put into practical use. The contact charging method contacts a charger to a photoreceptor and a predetermined bias is applied to the charger to charge surface of the photoreceptor. The contact charging method includes many methods using a roller, a fur brush, etc. for charging the photoreceptor.
Namely, the contact charging method uses a charge injector injecting a charge into the photoreceptor from the charger without a discharge phenomenon.
Specifically, the charge injector injects a charge from the charger into a charge holder such as a trap level or an electroconductive particulate material on the surface of the photoreceptor. The charge injector does not need a discharge, and a potential of the charged photoreceptor is proportional to a charging bias. Namely, even when a voltage applied to the contact charger is not greater than a discharge threshold, the photoreceptor can be charged to have a potential equivalent to the voltage. Further, the charge injector does not have an adverse effect such as image distortion due to a discharge product because of needing no discharge.
The charge injector needs to improve the contact between the charger and photoreceptor (firmly contact the charge to the surface of the photoreceptor) to improve the charge injection efficiency (uniformly charge the photoreceptor). However, the conventionally-used charger using a roller or a fur brush is difficult to firmly contact the surface of the photoreceptor due to reforming accuracy and wearing of the surface of the photoreceptor.
Contacting plural points of the charger to the photoreceptor is considered to improve the contact therebetween. Specifically, the traveling speed of the photoreceptor is differentiated from that of the charger at the contact point. However, it is difficult to contact a roller to the photoreceptor due to a friction therebetween. Although it is easier to contact a fur brush to the photoreceptor with a speed difference, the fur brush does not fully contact the photoreceptor.
Japanese Published Unexamined Patent Application No. 10-307454 discloses placing an electroconductive particulate material between a photoreceptor and a contact charger for the purpose of improving electrical contact therebetween, particularly facilitating the speed differentiation between a roller charger and a photoreceptor.
Japanese Published Unexamined Patent Application No. 10-307454 discloses a method of directly providing the electroconductive particulate material to a charger; Japanese Published Unexamined Patent Application No. 2000-81771 discloses a method of providing the electroconductive particulate material from an image developer; and Japanese Published Unexamined Patent Application No. 2001-242686 discloses a method of providing the electroconductive particulate material from a transferer.
Many methods of adding the electroconductive particulate material as an external additive are also disclosed. For example, it is widely known that a carbon black is added to the surface of a toner to impart conductivity thereto and control electrical properties thereof. Japanese Published Unexamined Patent Application No. 2-120865 discloses a method of adding an electroconductive particulate material comprising polyaniline.
The electroconductive particulate material adheres to or mixes in a contact charger and is placed at a contact point between a photoreceptor and the contact charger. The electroconductive particulate material fills a gap between the contact charger and the photoreceptor to improve the electrical contact therebetween even when the surfaces thereof are nonuniform. Further, the electroconductive particulate material works as a spacer to facilitate contacting the charger to the photoreceptor with a speed difference. Thus, since the electroconductive particulate material can maintain contact of the charger to the photoreceptor, the contact charger can well inject a charge into the photoreceptor.
However, even when a combination of the contact charger and electroconductive particulate material is used, defective images due to defective charge are produced because it is difficult to control adherence of the electroconductive particulate material to a photoreceptor and environmental resistance thereof.
Recently, an oilless fixer has been put into practice in terms of writability of recoding papers. A toner including a wax is typically used for the oilless fixer. Japanese Published Unexamined Patent Application No. 2001-34016 discloses a wet-type polymerized toner, which mostly includes a wax having a large particle diameter in the center thereof because the wax is difficult to minutely disperse therein. Therefore, the wax is difficult to exude on the surface of the toner from the center thereof even when a heat and a pressure is applied thereto, resulting in insufficient hot offset (to a fixing roller) resistance thereof.
Japanese Published Unexamined Patent Application No. 8-101526 discloses a toner, on the surface of which a wax is much exposed, effectively preventing the hot offset in the fixing process. However, a wax typically having a low softening point and hardness adheres to every part of an image forming apparatus, resulting in production of defective images.
Japanese Published Unexamined Patent Applications Nos. 2003-131416 and 2003-207925 disclose a toner on which a wax is present, which possibly has the same problem as the above-mentioned toner disclosed in Japanese Published Unexamined Patent Application No. 8-101526.
Particularly, in order to fully charge a one-component developer which is a toner, the layer thickness of the toner on a developing roller needs to be thin. This is because only the surface of the toner layer is charged, i.e., the toner layer is not uniformly charged when thick. An excessive mechanical stress applied to the toner layer accelerates exposure of the wax on the surface of the toner and causes adherence thereof to every part of an image forming apparatus such as a contact charger, resulting in defective images such as stripe images.
Even when the adherence is prevented such that stripe images are not produced, a typical wax has a resistivity (insulativity) higher than that of the contact charger, resulting in production of defective images due to defective charge.
Because of these reasons, a need exists for an image forming method which uses a toner including a wax, preventing adherence of the wax to every part of an image forming apparatus such as a contact charger, and production of defective images due to defective charge even when the wax adheres thereto.